For expository convenience, the present invention is illustrated with reference to the transmission of a disabling tone to an echo canceller apparatus on a digital communication circuit. However, the invention is not so limited and is useful in numerous other applications.
Echo cancellers are commonly used on long distance communication circuits to suppress echoes that may be caused by various delays and impedance mismatches along the path. Echo cancellers add greatly to the intelligibility of voice transmissions.
In certain circumstances, echo cancellers are undesirable. Such is the case, for example, when transmitting digital data. Accordingly, echo canceller apparatuses often include provisions whereby they can be disabled upon receipt of a predetermined command signal. In most systems, this command signal takes the form of a 2100 hertz tone.
An illustrative application of the use of this command signal is found in 300 baud computer modems. 300 baud modems generally encode the data being transmitted in frequency shift keying format. A space (logic 0) is transmitted as a 2000 hertz tone and a mark (logic 1) is transmitted as a 2200 hertz tone. A transmission of data at 300 baud using these tones includes a 2100 hertz component that is sufficient to disable any echo cancellers along the circuit. Thus, 300 baud modems act automatically to disable any echo cancellers that may interfere with their accurate transmission of data.
In other data transmission applications, a 2100 hertz tone can be transmitted continuously with other data tones. In still other applications, a 2100 hertz tone can be transmitted for a few hundred milliseconds when the data link is first established and discontinued thereafter. Both techniques have been used to disable echo cancellers on digital circuits.
Most of the prior art systems employing the disabling feature of echo cancellers are analog communications circuits in which data is encoded as a series of audio tones. A more difficult situation arises on true digital lines in which "1"s and "0"s are transmitted directly, without being first encoded into tone form. Exemplary of such systems is the Accunet.RTM. 56 kbit/sec. service offered by AT&T. In this system, the only means of transmitting the 2100 hertz tone to the echo canceller is in digital form. That is, the analog 2100 hertz tone must be digitized and the digitized samples then transmitted serially along the circuit. In an exemplary system, the samples are in the form of 7-bit bytes.
The problem encountered in disabling echo cancellers on purely digital circuits is that it requires knowledge of the byte boundaries used by the digital network to which the echo canceller is coupled. This information is generally not available to end users of the system. End users simply apply serial data to the circuit without regard to the framing information that is later added to the data by the network. (In networks such as the Accunet.RTM. 56, serial data is sent without any framing information from the user to the switching office. A digital switch at the switching office then frames this serial data stream into bytes for further transmission by adding an eighth signalling bit after every seven bits received from the user. Th data then maintains this byte-organization until it is relayed finally to the receiving station, at which time the framing information is stripped off and an undelimited string of serial data is provided.)
The echo cancellers of concern are in the byte-framed portion of the digital network and thus require that the digitized 2100 hertz tone have the proper byte boundaries. Since the originating station operates without reference to the framing boundaries employed by the network, the originating station has heretofore been unable to disable the network echo cancellers.
To disable the echo cancellers on digital networks, it has previously been necessary to call a network operator and ask the operator to disable the echo cancellers. The operator is able to perform this operation because the telecommunications carrier can introduce the digitized 2100 hertz disabling signal into the network after it has been framed into known eight bit bytes.
This technique of calling the operator whenever disabling of the echo cancellers is desired is unsatisfactory. The only alternative has been to lease dedicated lines that do not include echo cancellers. This option, of course, is expensive.
Accordingly, there remains a need for a technique to permit byte-organized data to be sent to a byte-synchronous receiving system from an originating station that has no information about the framing boundaries employed by the receiving system.
Consequently, it is an object of the present invention to permit the successful transmission of byte-organized data from an originating station to a receiving station without regard to the byte boundaries employed by said stations.
It is a more particular object of the present invention to permit the disabling of echo cancellers on digital networks by transmission of control signals from originating stations that have no information about the framing boundaries employed by the echo canceller.
According to one embodiment of the present invention, an apparatus associated with the originating station on a digital data network transmits a burst of digitally encoded 2100 hertz tone to the network for a brief period, such as 500 milliseconds. The data comprising the burst is then bit-shifted one bit relative to the prior transmission and is retransmitted. This process is repeated until bursts of the 2100 hertz data have been transmitted with each possible alignment of the byte boundary (i.e. until bursts of the data have been transmitted N times where N is the number of data bits in a byte.) The byte boundaries employed by one of these bursts will coincide with the byte boundary employed by the digital network of which the echo cancellers of concern are a part. Thus, one burst will be successful in disabling the echo cancellers without intervention of an operator and without resort to use of dedicated lines.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the detailed description, which proceeds with reference to the accompanying drawings.